2 002 IEEE, Short Term Braking Capability

8/3/2019 2 002 IEEE, Short Term Braking Capability

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Short term Braking Capability during Power Interruptions forIntegrated Matrix Converter-Motor DrivesChristian Klumpner Frede BlaabjergAalborg University, Institute of Energy TechnologyPontoppidanstradae 101, DK-9220 Aalborg East, DENMARKPhone: +45-9635 9283, Fax: +45-9815 1411,Email: [email protected], [email protected]

Abstract - The new trend in A djustable Speed Drives (ASD)is to integrate the inverter and the motor into a single unit inorder to reduce the produc tion cost, the com missioning timeand the ph ysical size of the equipment. This last issue becomesmore important, making the matrix converter topology moreattractive. Sinusoidal input c urrents and bi-directional powerflow are other advantages of the m atrix converter but it is lessimmune to power grid disturbances compared to a standardASD. In hoisting applications, short-term braking capabilityduring a power outage is needed until the mechanical brakeengages or to perform mor e effective a combined braking.This paper proposes a new method to provide short-termbraking capability during a power outage for matrixconverters. A braking chopper is needed in the clamp circu it,which allows for a drastically reduction of the capacitor sue.The power flow in the clamp circuit may be reduced byincreasing the harm onic content in the motor currents, whichcauses h igher motor losses. Experiments prove the validity ofthe proposed method.I. INTRODUCTION

Bi-directional power flow drives, like the back-to-backVoltage Source Inverter (VSI), which already iscommercial, have inherent long-term full braking torquecapability, as long as the quality of the power grid allows.This is desired in applications where regenerating powerimproves the overall efficiency as in elevators, cranes, anddownhill transportation systems. However, during voltagesags or during momentary power interruptions, the brakingcapability of a drive is seriously affected, and in order tocomply with safety regulations extra hardware (brakingchopper) is needed [ l].Limited braking capability for a unidirectional powerflow ASD, without employing a braking chopper, isnecessary in order to provide a low-cost solution forapplications that require only a reduced braking torque.There are several techniques to achieve this, but allmaximize the losses in the motor and the converter. In [2],higher losses have been achieved by increasing the fluxreference above the rated level during decelerations n orderto limit the increase of the DC-link voltage level while theconverter was used in synthetic loading of an inductionmotor. In [3] a critical braking method is proposed, byoperating the motor at a slip level where the regenerativepower and the power dissipated in the windings are equal.However, this method causes high currents, which require ahigher rating for the semiconductor components. In [4] acomprehensive analysis of the motor losses in bothmotoring and regeneration mode is done, the traditionalbraking methods are presented and a novel braking method

is also proposed. This consists of modulating the d-axiscomponent of the motor current in a way that increases itsRMS value but maintains constant its average, causing anincrease of the iron and the copper losses. All thesemethods may be applied during power interruption becausethe diode rectifier will disconnect the DC-link from thefaulty power grid, while the control algorithm maintains theDC-link voltage within a working range by controlling thatthe braking power is equal to the losses in the drive.A Direct Frequency Converter (DFC), as it is the case ofthe matrix converter, consists of a single power conversionstage. Because of the lack of reactive elements, thistopology has a great potential for size reduction, andintegrated motor drives with bi-directional power flowcapabilityare a possible target [5]. Because it has no energystorage components, it is impossible to control the motorduring a power outage. Moreover, braking becomes aproblem because it is not possible to evacuate the electricalpower. In [6]-[7] it was shown that it is possible to provideDFCs with ride-through capability, only to feed the controlelectronics for short-term (hundreds of milliseconds), aslong the motor flux which is impossible to control stillexists. This control strategy was producing anelectromagnetic braking torque of 10-20 % of the ratedmotor torque, which reduced the speed of the motor.

This paper investigates the possibility to use the sameprocedure to control the motor, while the main target is togenerate a higher braking torque for a short term during apower interruption. Because the power flow is considerablehigh, a braking chopper is necessary to be connected in theclamp circuit. It is possible to maximize the motor losses byincreasing the harmonic content on the motor currents.Even though adding an active switch increases the ASDcomplexity, it may drastically reduce the size of the clampcapacitor when the chopper is designed to handle the worst-case fault currents. The braking resistor has to be able toaccumulate this energy because the usage of the chopper isintended for short-term usage (emergency situations).Requirements to handle the braking energy flow may bereduced by maximizing the motor losses. This is achievedby increasing the harmonic contents of the motor currents,as the motor flux cannot tie controlled. The method isexperimentally evaluated.11. PRACTICAL TOPOLOGYF A MATRIXCONVERTER

The most important advances in the matrix convertertechnology have been achieved in the last 20 years. Theseinclude: modulation techniques to provide sine-wave-insine-wave-out operation [8]-[111, safe commutation

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.otatingvectoi(synchronous]

Rotatingvectors(inverse)

Activevectors(pulsating)

( 4

Zero-vectors

Fig. 1. Basic topology of a matrix converter:(a) electric scheme; @) symbol; (c) permitted switching states(27) in a three-phase to three-phase matrix converter.strategies for bi-directional switches [121-[13], methods tocompensate the effect of unbalanced supply voltages [141,investigations of the protection issues [15]-[16] and of thepotential application such as the integrated matrixconverter-motor drive [SI. A few papers [17]-[21]investigate future trends in power electronics and gavecredit for the hture development and industrialimplementation of the matrix converter technology, as thetrend now is towards improving the interaction with thepower grid, providing bi-directional power flow, increasingthe efficiency of the drive while operating at higherswitching frequency, decreasing the drive size; integratingmore complex silicon structures in power modules, whichall match with the matrix converter profile. There are a fewdisadvantages as a voltage transfer ratio below unity and alow immunity to grid disturbances.A matrix converter consists of nine bi-directionalswitches, arranged into three groups of three, each groupbeing associated with an output phase. This arrangement ofbi-directional switches connects any of the input lines a,or c to any of the output lines A, B or C, as it is shown inFig. la. The symbol, which is often used in a graphicalrepresentation, is presented in Fig. lb, where a filled circlerepresents a closed switch. The 3x3 switches give 512combinations of switching states, but only 27 permittedstates remain if the two basic rules to operate this convertersafely are applied:

DO NOT connect two different input lines to thesame output line (short-circuit of the mains causesovercurrent);DO NO T disconnect the output line circuits(interrupt inductive loads causes overvoltage);In Fig. 2, a practical topology of a matrix converter isshown. An input L-C filter is necessary to reduce the high

line motorFig.2. A practical scheme of a matrixconverter drive with aninduction motor (Ih4).

frequency ripple fiom the input current. For protectionpurposes, it was shown [151 that a clamp circuit is neededto provide safe shutdown of the converter during faultysituations as overcurrent on the output side or voltagedisturbances on the input side. A Switched-Mode PowerSupply (SMPS) is connected to the clamp circuit to feed thecontrol circuits of the converter.111. BRAKINGMETHOD URING OWER INTERRUPTIONSA method to achieve low-cost braking capabilities forunidirectional drives is to increase the motor losses duringregeneration. Methods to increase the iron and copperlosses have been investigated in [2]-[4]. Todays frequencyconverters include software features as DC-braking, flux-braking etc, to provide low-cost braking of the drivewithout requiring extra hardware when light-load long-termbraking capability is required. Moreover, during faulty gridconditions, short-term full torque braking capabilities areavailable, due to the energy storage capacity in the DC-link.The matrix converter provides full braking capabilityduring normal power grid conditions, regenerating thebraking power. Therefore, none of the features mentioned

before are needed in normal operating conditions. However,during faulty grid conditions, there is no destination for thepower regenerated by the drive, and therefore, it is notpossible to obtain long-term 4-quadrant operation. Thedrive is forced to shutdown and the control of the motor islost (free running). In the case of a critical application, anemergency brake stop of the motor shaft should occur,which require a mechanical brake. There is a delay of tensof milliseconds between the moment a fault signal isproduced and the release of the mechanical brake whereelectrical braking may be useful. Some drive manufacturersoffer the possibility of a combined electrical andmechanical braking, which helps in decreasing the stoppingtime and the stress on the mechanical brake. In this case, i sno need for long-term regenerative operation and electricalbraking may start immediately to reduce the stress on themechanical brake. These are the reasons to investigate thepossibility of short-term braking capabilities for matrixconverters, since a possible application in integrated motordrive may apply to cranes and elevators, which requireemergency stop during faulty situations and employmechanical brakes due to specific safety regulations.

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@)Fig. 3. Scheme of the matrix converter and conduction path of the motor currents when the allowed switching ;states are applied: (a) meve ctor ; (b)disconnection of the active switches and diodes hewheel ing in the clamp circuit; (c) the energyflowof the proposed energy conversion procedure usedto brake a matrix converter and (d) transition ffom normal toproposed operation. U,, (100 V/div), I,, (5 Ndiv) are he motor voltage and current,

nmt(170 rpdd iv) is the motor speed and U, 100 V/div) is the voltage in the clamp circuit.The requirements for short-term braking of a matrixconverter should be analyzed and what kind of extrahardware is needed. Since this should operate in

conjunction with a mechanical brake, electrical brakingshould start immediately when a power grid failure isdetected and it should be able to continue operation as longas possible, without the risk of causing a hardware failure.A delay for the mechanical brake reaction of 5-30 ms and abraking time of 100-500 ms may be taken into account,since the braking torque of a mechanical brake is usuallynot higher than 2-3 times the rated motor torque in order toavoid damage of the gear.As explained in [6]-[7], during a total power interruption,only switching states that provide separation of the motorcircuit fkom the power grid circuit are allowed. These are:0 zero-vectors (ma, bbb, or ccc) that connect allmotor terminals to the same line of the power grid, as it isshown in Fig. 3a. Because the rotor flux and the rotor speed

are not zero during operation, his short-circuitof he motorwill force the stator currents to increase.0 disconnect the switches, so the motor currents willflow through the clamp circuit. This situation is shown inFig. 3b. Because the voltage in the clamp circuit is higherthan the magnitude of the back EMF of the motor, thisaction will force the stator currents to decrease.By alternating the two permitted switching states, amechanical-to-electrical energy conversion procedure takesplace, as long the decaying rotor flux allows. In Fig. 3c isgiven the energy flow path. During a zero vector, thecurrents increase and the mechanical energy is convertedinto magnetic energy stored in the motor leakageinductance. When the switches are disconnected, themagnetic energy is released into the clamp circuit. This

method was extendedly presented in [7 ] and used only torecover a small amount of energy to feed the controlcircuits (see Fig. 2). Now the requirement is to brake withmaximum torque capability in respect to the matrixconverter current rating. Therefore, the power flow in theclamp circuit is expected to reach the rated motor powerlevel. In Fig. 3d the waveforms of the motor voltage,current, speed and the voltage across the clamp circuit areshown, when the transition from normal operation to the

proposed braking operation occurs. A braking choppermounted in the clamp circuit maintains the voltage level toa safe level (650 V).The braking process shoulid not rely on the grid voltageand therefore, a device which is able to dissipate thebraking energy should be implemented in the matrixconverter. As it was already shown [151, a clamp circuit isneeded in a practical topology in order to store energyduring faulty situation, and therefore it is chosen to connecta braking chopper in the clamp circuit. This solution allows

to decrease the physical size of the clamp capacitor since itis cheaper, safer and more compact to convert the electricenergy caused by shutting down the converter during afaulty situation into thermal energy, store it in the body ofthe braking resistor by increasing its temperature andrelease it gradually, han the electrical storage in capacitors.In Table I are given few values for the heat capacity ofsome materials. In Table I1 are given the characteristics ofthe clamp capacitor chosen in the design of a 4 kW MatrixConverter-Motor (MCM) [5].

T,BLE I.HEAT APAIS~IYN MATERIALS

Material Heat capacity ( Jk gK)Water 4190Ceramic = 880Aluminum 890Iron 465copper 390

TABLEI.CHARACTERISTICSFTHECLAMPAPACITORNTHE MCM PROPTOTYFTElectricalparametersStored energy@lo00VSize 4 2 . 5 ~ 4 6 5 ~ 5 5 Mass/ Volume 0.15 kg109 cm3

25 @/950 V, 00V/ps12.5 J

In case the electrical energy in excess during faultsituations is converted into heat, it is possible to comparethe thermal energy storage versus the capacitive energystorage. If the total amount of energy stored in a 25 pFcapacitor at 1000 V is converted into heat, it will increasethe temperature of 0.001 kg ceramic material by 14.2 K.Because is intended only for faulty situations, the thermal

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energy is then lost gradually, so a heatsink is not required.It is possible to remove the clamp capacitor by sizing theresistor and the switch in the braking chopper to handle theovercurrent level Zfault while the voltage across the bi-directional switches seen also in the clamp circuit is belowthe maximum level Uma:Rcho,,,, U , /'fm/, (1)A similar approach to dissipate the energy during a faultin a matrix converter was presented in [16], implyingvaristors and active gate-drivers to operate the switches inthe linear region, without the need of extra power devices.

IV.METHODO MAXIMIZEHEMOTOR LOSSESIt is possible to decrease the energy flow into the clampcircuit by maximizing the motor losses. In [6] it has beenshown that no reactive power is exchanged with the motor,but current harmonics of higher magnitude are producedwhen the two types of switching states are used. It is knownthat the resistance of the motor windings is changing withthe frequency due to the skin and proximity effects. In Fig.

4 is shown the stator-phase resistance dependence of thefrequency in the range 20 Hz - 10 lcHz of a 3 kW inductionmotor, measured with a digital RLC-meter. Therefore, bymaximizing the magnitude of the high-frequency ripple inthe motor currents, it is possible to cause additional lossesin the induction motor at a similar RM S value of the motorcurrent. In Fig. 5a, the equivalent scheme of the matrixconverter drive is shown during the proposed brakingmethod and this may be split in two models. The model forlow frequency is shown in Fig. 5b, where the matrixconverter produces a sinusoidal voltage synchronized withthe back EMF of the induction motor, which now workssimilar as a synchronous generator [ 6 ] . A low-frequencythree-phase sinusoidal current in phase with the phasevoltage is drawn and because the stator winding resistanceis low, most of this active power is transferred into theclamp circuit and dissipated in the braking chopper. Themodel for high frequency is shown in Fig. 5c, where thematrix converter is producing a square-wave voltage ofvariable duty-cycle and a peak-to-peak magnitude given bythe clamp circuit voltage. The equivalent scheme of theinduction motor consists of a single-phase connection of thethree windings, as shown in Fig. 5c. As the harmonicspectrum of the voltage is above the switching frequency, itis expected that most of the losses caused by highfrequency currents, take place in the motor.A method to estimate the losses in the motor is to use thesuperposition principle of the low and high fiequency140- 20

si 10080. 602 40

201 00 1000 Freq. [Hz]Fig. 4. Stator-phase resistance dependencyoffrequency for a 3 k W induction motor.

0) (c)Fig. 5. The ASD scheme diagrams during the proposed brakingmethod: a) Electrical single-phase diagram;b) Low frequency m odelU=$):issipationof the braking energy is mainly in the chopper, c)High frequency model cf> fw):dissipationof the braking energy ismainly in the statorwindings(Rs).

effects. A model to verify the importance of the highfrequency losses is proposed by considering the voltageproduced by the matrix converter as an ideal square-waveof 50% duty-cycle and a peak-to-peak value given by thevoltage in the clamp circuit. The power loss in the statorwindings may be easily determined considering the currentharmonics through the equivalent resistance (Fig. 5c):- -

I,"=213.U:/( R,"+ k. .x .f, .Lt )

where: I:, U: are k order harmonic RMS value of thecurrent and voltage high frequency ripple, RF , Ls: are thefrequency dependent resistance and leakage inductance ofthe stator phase winding,fw is the switching frequency ofthe square-wave voltage and P,,-l,,ss is the amount oflosses caused by the high-frequency ripple harmonics in themotor.The model implemented in Matlab considers a constantstator leakage inductance of the induction motor of 7 mHand a variable dependence of the stator resistance as givenin Fig. 4.Harmonics up to 7* order are taken into accountwhen calculating the losses ( 2 ) . Comparative results areshown in Fig. 6a and Fig. 6b in two cases: constantresistance of the stator winding of 3.87 SZ (RFigA@ 50 Hz),and variable with the frequency (Fig. 4). While the RM Svalues of the high-frequency current ripple are very similar(Fig. 6a), the losses caused in the stator winding are muchhigher for a variable dependence of the stator resistance(Fig. 6b) and this difference increases with the frequency.However, as the duty-cycle is varying in time and it isdependent on the control method of the motor currents, amore accurate model to estimate losses is hard to develop.The magnitude of the current ripple (2) is limited by thefollowing factors: the clamp circuit voltage, the switchingfrequency, the stator resistance dependence with thefrequency and the stator time constant which gives thedildt. As it was shown in [6]-[7], the switching frequency is


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fm00 800 1MO 1200 1400 16 M 1800Fig. 6. Comparison ofvariable vs. constant stator resistance of the

stator winding on he motor losses: a) the high-frequency c m t ipplein the stator windings; b) the losses in the stator windings. Harmonicsk={1,3,5,7};Uclamp= 650 V, R-*@ 50 Hz, s - ~ 7 H.

influenced by the limit for the motor current vector whenthe matrix converter changes its state and again, by thestator time constant L,dR,. It is possible to influence thecurrent ripple by the type of the current controller: ideal bi-positional (bang-bang) or bi-positional with hysteresis.This is shown in Fig. 7 where the effect of two type ofdiscontinuous control on the motor current is presented. InFig. 7a, the effect of using the bang-bang control [6]-[7] isshown, while in Fig. 7b, the effect of the hysteresis controlcause higher current ripple while similar RM S value is keptconstant, but the frequency of ripple is lower.It is important to mention that the braking capability isvery sensitive to th e induction motor parameters, andguarantees to develop a certain braking torque for anymotor without testing cannot be given. Therefore, thismethod is more suited for applications where the motor andthe matrix converter are designed as a single unit, as is theMCM[5].v. EXPERIMENTALVALUATIONF TH E BRAKINGMETHOD

The experimental setup consists of a 8.5 kVA matrixconverter, where the bi-directional switches consists ofcommon-collector connected 1200 V/25 A IGBTs. Thesampling/switching frequency in 7 kHz. In normalN-1 N N+l

I I I I I I I II I I I I I I II I I I I I I IZ1kWl l 1 1I I I I I I I I

I I I 1i i i i i i i i

( 4 @)Fig. 7 . Current shape controlwith he proposed braking method when(a) bang-bang control is used and @) when hysteretic control is used .

operation, Space Vector Modulation (SVM) s used toperform the control. More details about the implementationof SVM are presented in [7]. The matrix converter isdriving a 3 kW nduction motor, mechanically connected toa DC-machine, which is used as a mechanical load. Theelectrical parameters for the matrix converter prototype andinduction motor are presented in Appendix A and B,respectively.In Fig. 8, the steady-state voltage and current waveformson both sides of the matrix converter are shown. The lineside performance is excellent, as the drive draws sinusoidalline current in phase with the input phase voltage. In Fig. 9,the inherent bi-directional power flow feature of the matrixconverter is shown. The output frequency of the matrixconverter is maintained constant (20 Hz) and the transitionfrom motoring to generating is performed by disconnectingthe rotor circuit of the DC-imachine used as a mechanicalload fi-oma resistor and connecting it to a DC supply. TheDC machine changes from generator operation to a motoroperation, when ZDcgen changes sign and accelerates abovethe synchronous speed of the motor. When the inductionmotor losses are fully covered from the rotor side, the linecurrents of the converter are purely capacitive, but as themotor speed increases, more power is coming out from therotor and is delivered to the grid, which is denoted by theopposition of the input phase voltage and the line current

Fig. 8. Steady-state performance of a matrix converter drivinga3 kW induction motor at 35 Izand 50 YO f rated torque. Inputphase voltage U,. (160 V/div), Line currentI, (5 Ndiv), Outputphase voltage U,, (200 V/div), Motor current I,, (5 Ndiv).Time scale: 4 mddiv.

Fig. 9. Line side performance when a transition !?om motor togenerator occurs. Input phase voltage U,, 160 V/div), Line currentIj,, (2 Ndiv), Current in theDC: oadmachine 1 ~ ~ 4 5div). Timescale: 20 ms/div.fmt=20 Hz, L,, = +30 % TN, L ~ ~-60 % TN.

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Ch 3 2 O O VFig. 10.Acceleration test from 10Hz o 38 Hz:Input phase volta ge U, 160V/div), Line current Ii,,(5 Ndiv),Motor current Zmn (10Ndiv). Time scale: 4 mddiv.

waveforms. In Fig. 10 an acceleration test of the matrixconverter drive from 10 Hz to 38 Hz is shown. A constantresistor in the rotor of the DC machine gives a load torquethat is linear dependent with the speed: 15 % of rated motortorque at 10 Hz and 60% at 38 Hz respectively. Additionaldynamic torque due to inertia appears during acceleration(120 Hds), making the line current to reach 10A amplitude(7 A-), while the motor current reaches 15 A amplitude(10.6 A-).

A. IdentrfLing the Inertia of the Mechanical SystemA deceleration test, while the DC-machine is connectedto a constant resistive load is carried out to determine theinertia of the system. Because the armature current isvarying linear with the back EM F (rotor speed), it may beconsidered that the load torque profile is linear andtherefore, the rotor speed decelerates exponentially(3).

T , = 0; (3)(4)

where: J is the inertia of the two rotors, Tem is theelectromagnetic torque of the induction motor, SZ is theangular speed of the motor, TL(SZ) s the load torque, kT isthe DC-load machine constant given by its resistive loadand Tmech is the mechanical time constant of the two rotors.This is shown in Fig. 11,where the induction motor wasinitially running at 30 Hz, nd the DC-machine provided 50% of the rated torque of the induction motor. The matrixconverter is turned off and the deceleration process isrecorded. It was found that Tmech=0.41 s and J= 0.034kgm.B. Parameters U sed to Evaluate the Braking MethodThe average electromagnetic torque produced by theproposed braking method may be estimated from thedecrease in the rotor speed using (3), considering that theinduction motor was initially running at no load:

(5)where Qini and Q, are the angular velocity of the motorat the beginning and respectively at the end of braking,p is

Te

i476mV---- M looms A

IU- 133 200msFig. 11. Deceleration test to estim ate the inertia of the motor.Tangent to n,, curve gives T,,,=I,= 0.41 s.the number of poles of the induction motor,f* is the initialfrequency, and n d n k i is the relative speed drop.The braking energy distribution is important to estimatethe effectiveness of the braking method, as smaller energyflow in the clamp circuit may reduce the electricalrequirements for the chopper components. This is done bycalculating the efficiency of converting mechanical energyinto electrical energy as a ratio between the energydissipated in the braking chopper versus the mechanicalenergy taken from the inertia during braking. If in a normaldrive system, a higher efficiency is desired, in this situationa poor efficiency will allow for a lower rating of thechopper components. The mechanical energy taken fromthe inertia is given by:

The electrical energy dissipated in the braking resistor ismeasured by monitoring the current that flows in thebraking chopper and the voltage in the clamp circuit. Themultiplication math function of the oscilloscope is used tocalculate the instantaneous power, and the mean value isused to determine the electrical energy dissipated in thechopper. A correction is necessary, considering the energyaccumulated in the clamp capacitor before the chopperstartsoperating.

(7)L

where Pma is the quantity displayed by the oscilloscopecorresponding to the mean value of the dissipated power inthe braking resistor, Cc,,, is the value of the clampcapacitor, Vi, U, are the voltage in the clamp circuitbefore braking and the mean value after the brakingchopper starts to operate.The average power-losses in the motor may becalculated:

C. Current Spec trum Influence on the Loss DistributionThe motor current, the motor speed, the voltage in theclamp circuit and the current that flows in the brakingchopper are measured in order to determine the balance

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Fig. 12. Testingof the proposed braking method in two situations:bang-bang motor current controlAi,,,= '7A (left sid e); hysteresis motorcurrent control 10A, Z I ~ ~ - ~0.5 A (right side). (a), @) motor speed n (160 r pd div ), motor currentImn 5 N div ) and its FFT 1Nd iv, 625 Wd iv). (c). (d) voltageinthe clamp circuit Ua- (100V/div), power dissipated in chopperpa,,, (2 kW /div) and the currentinthe braking resistorIBR5 Ndiv). Time scale: 4 m s/div. Test c0nditions:fbi = 40 Hz,& = 7 kHz.between the mechanical energy taken ftom the rotor inertiaan d the electrical energy dissipated n the braking resistor.The test consists of braking against the inertia of theinduction motor, which is coupled with a DC machine. Asit was shown in Fig. 4, the measured resistance of the statorwindings varies much with the frequency. Therefore, it isexpected that by changing the motor current spectrum, thelosses in the motor it will also change. In Fig. 12,tests havebeen carried out imposing similar RM S current that ideallywould produce similar motor losses. The effect of the bang-bang and the hysteresis motor current controllers, presentedin Fig. 8a and Fig. 8b, are compared. In Fig. 12a and Fig.12c, the shaft speed, the motor current and its FFT havebeen shown. The calculated braking torque T,,,,.,, is similarand the level is significant, 71 % of the rated motor torque.The FFT of the currents show a higher current ripple in thecase of the hysteresis control, while the order of the highestmagnitude harmonic is lower. In Fig. 12b and Fig. 12d areshown the voltage in the clamp circuit, the current in thebraking resistor and the dissipated power. Its mean valuegives the braking energy in the clamp circuit (6). Table I11summarizes the tests results. These two situations areequivalent regarding the RM S motor current, while thefundamentals 1, and the most important harmonics 4 ,differ significantly. By causing higher ripple of the motorcurrents, the energy flow in the braking resistor issignificantly reduced, from 71 % to about 60 %, whichallows for lower power dissipation requirement for thechopper. However, the current peak to be handled by theswitches is higher in the second case (12 A vs. 10 A),

which causes higher stress i n the semiconductors, but stillfar below the hardware protection limit (20 A).T P ~ L EII.

COMpARAnvE EFFECT ANALYSIS OF TW O BRAKING ETHODSParameter Bang-bang control Hysteresis controlT (5) 14.3 Nm (71.5 %) 14.3 Nm (71.5 %)ndn,, 0.132 0.132Wtah (6 ) 66 J 66 JIrurcs 4.68 A 4.79 A3.56 A @ 37.5 Hz 3.2 A @ 37.5 HzI k - m 0 . 7 4 A @ 1 .3 kH z 1 . 8 4 A a 6 2 5 H zpchopper 1147 W 951 WW e l q 0.63 J 0.82 JWel-ma (7) 46.85 J 39.35 J

Weycl-motlWmcch 71 % 59.6 %Pm,-lmr (8) 468 W 645 W

As a design rule the braking resistor should handle thepeak current safely and whim the maximum level of thebraking energy (9) is converted into heat, it should notexceed its maximum operating temperature (10).(9)BR-,, s R.TN ,,,, . N 130

7= WB R - - .mm (10)This is checked for a :3-kW induction motor drive(parameters given in Appendix B), which has to break for200 ms at its rated mechanical torque and speed giving abraking energy of 605 J. In the extreme case that all thisenergy turns into heat in the: body of the braking resistor,which may consistsof 0.015 kg ceramic (cBR = 880J k g K ) ,its temperature will raise by 415.8 K, which is acceptable.

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W. CONCLUSIONSThis paper proposes a new method to provide short termbraking capability for a matrix converter during powerinterruptions. A braking chopper is needed, but because it isintended for short-term operation, this is not bulky. Thebraking energy is converted into heat, accumulated in thebraking chopper resistor and released gradually. It is proven

by measurements that it is possible to maximize the motorlosses by increase the harmonic content on the motorcurrent, allowing a lower rating of the chopper elements. Itis suggested that the braking chopper allows for sizereduction of the clamp capacitors, as storing the energyduring faults thermally is more compact than electrically.APPENDIX

The parametersof the matrix converter prototype:S= 8.5 kVA, Uh= 3x380 V, him = 21beakC-,= 6 PFI 250V cy), i, = 0.7 mW10 A, CO,, = 14.5 pF/lOOO V, Rg =120Q.APPENDIX B

The parameters of the induction motor used in the test:P N = 3 kW, U* 380 V, IN = 6.9 A,fe 0 Hz, n e 430rpm, 7~ 81 %, coscp= 0.82; J= 0.0082 kgm2,T e 0 Nm,R,= 1.790,a-s= 7mH,Rr= 1.8Q, Lgr= 14.4mH,L,= 160mH.

ACKNOWLEDGEMENTThe authors acknowledge the financial support (contractno. 2013-01-0045) to the project by the Innovation Post.Doc. programme at the Danish Research Council and thesupport from Danfoss Drives A / S .

REFERENCES[l ] J. Rodriguez, . Pontt, A. Weinstein, Regenerative drives in themegawatt range for high performance downhill belt conveyors,

P. Lesage, M . AlaWla, L Gertmar, The Dynamic ThermalLoading of an Induction Machine, Proc. o f EPE97, vol. 2, pp.M. Nomura, H. Ikejima, S. Morita, E. Watanabe, Regenerativepower control for VWF motor drive (critical braking methodapplied to the elevator), Proc. o f APEC88, pp. 97 -105, 1988.J. Jiang, J. Holtz, AnEIficient Braking Method or Controlled ACDrives with a Diode Rectzjier Front Ens, Proc. o f IASOO, vol. 3,

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